While on hold, such as while waiting in a hunt group (also referred to as a queue) of a contact center, a party to the call that has been put on hold may hear silence, music, or periodic reminders that the call is important and to stay on the line. In some cases, voiced announcements to the on-hold party provide estimates of the wait time.
One disadvantage of these arrangements is that the on-hold party must listen on the call for an indication that the far-end resource has taken the call off of hold and come on line. Having to listen to the call while waiting on-hold inconveniences the on-hold party and interferes with that party's ability to put the waiting time to good use.
Another disadvantage of these arrangements is that, in some scenarios, large amounts of network bandwidth and resources are utilized by on-hold calls. For example, if the audio encoder specified by ITU-T Recommendation G.711 is employed for the music-on-hold or for the status announcements (this being the most commonly used audio encoding algorithm for telephony applications), a transmission rate of 64,000 bits per second is required.
In packet-switching communications systems, particularly Voice over Internet Protocol (VoIP) systems, various standards organizations have proposed IP mechanisms by which voice and conversational text can be intermixed in the same phone call. For example, ITU-T Recommendation T.140 and RFC 4103 describe a mechanism by which voice and text are intermixed. Additionally, concurrent intermixing of text and voice is supported by the teletype (TTY)-on-VoIP architecture, in which text is transported on VoIP networks as RFC 2833-format descriptions of the corresponding Baudot TTY tones. Control signals and voice stream data is conveyed along the same channel in-band by using different types of packets (i.e., using different packet headers).